The invention relates generally to air core gauges and more particularly to a method and system for driving such gauges.
Air core gauges are typically driven in response to predetermined current levels applied to two or more coils. These coil currents create a magnetic field which causes a gauge pointer magnetically coupled to the coils by a magnetic rotor to align itself with the resultant magnetic field vector. Positioning of the pointer is controlled by varying the direction and magnitude of the current in the coils.
Several methods of driving the coils have been heretofore known. For example, it is known to drive cross-coil type gauges by varying the magnitude and direction of the currents in the coils. It has similarly been known to use pulse width modulation (PWM) to drive both cross-coil and wye-winding type air core gauges. The magnitude and direction of the currents through the coils in such systems are controlled by varying the duty cycle of the PWM signals.
Theoretically, gauge drive strategies employing PWM should be able to provide any degree of pointer resolution desired for a given application. In practice, however, gauge resolution is limited by competing hardware and software constraints which often must be compromised for optimum system performance. For example, a faster software loop allows for the gauge pointer position to be updated more frequently which results in better pointer resolution. However, the software interrupt time required to obtain the desired resolution may not permit enough time for all of the operations necessary to update the pointer position to be performed.
The limitations in pointer resolution caused by system hardware and software design constraints are particularly noticeable in the upper end of a nonlinear gauge's range of motion. For example, if a given PWM drive strategy is constrained to a duty cycle step of 200 microseconds and a drive period of 12.5 milliseconds, the drive resolution (duty cycle step/drive period) will be limited to 1.6% across the full range of motion. In some nonlinear gauges, this 1.6% resolution results in 3 degrees of pointer motion between duty cycle steps at the lower end of the range of pointer motion. However, the same 1.6% resolution will result in 6 degrees of pointer motion between duty cycle steps at the upper end of the range of pointer motion. Therefore, if 6 degrees of pointer movement is beyond the acceptable limit for the given application, the system will not be able to provide the desired pointer resolution.
A need therefore exists for a method and system for driving an air core gauge to provide improved pointer resolution given system design constraints.